By the Numbers

4,600,000,000Comets began to form about 4.6 billion years ago, at the same time as the outer planets Jupiter, Saturn, Uranus and Neptune. The sun was just becoming a star at this time.

1,000,000,000About one billion particles of comet dust - each the size of a particle of cigarette smoke - fall to Earth every second. This occurs because a comet's dust tail and coma continuously shed dust into space. As Earth circles the sun, it plows into this dust.

360,000,000The ion tails on comets can be huge. Comet Hyakutake's ion tail stretched 360 million miles. That's nearly four times the distance from the Earth to the sun, and one of the longest ever observed by humans.

4,000Scientists have discovered about 4,000 comets to date. However, they believe there could be hundreds of millions they have never observed in the Oort cloud.

5When NASA's EPOXI mission flies past comet Hartley 2 on Nov. 4, 2010, it will be only the fifth time humans see a comet up close. When NASA's EPOXI mission flies past comet Hartley 2 on Nov. 4, 2010, it will be only the fifth time humans see a comet up close.

Comet Facts for Students

Comets are extremely old, icy and oddly shaped objects in our solar system. They are exciting for a few reasons:

-- Since ancient times, humans have observed comets in the sky. Roughly once every 10 years, we can see a comet from Earth with the unaided eye. Its impressive tail blazes across the sky over the course of several days or weeks.

-- Scientists think comets hold pristine materials within them that date back to the formation of our solar system.

-- Many scientists believe that comet impacts delivered water and other important building blocks for life to the early Earth.

Why do comets have tails?

Comets only have tails when they are close to the sun. When they are far from the sun, comets are extremely dark, cold, icy objects. The icy body is called the nucleus. Nuclei are made of various types of ices, dirt and dust. As comets get closer to the sun in their journeys through the solar system, they start to warm up.

As they reach an area roughly the same distance from the sun as Jupiter, the ices start to heat up and vaporize, releasing the gases and embedded dust particles that form a cloud or atmosphere -- called a coma -- around the comet. As comets continue traveling closer to the sun, the dust particles and other bits of debris in the coma are blown away from the sun due to the pressure of sunlight. This process forms a dust tail.

If this tail is bright enough, we can see it from Earth when the sunlight is reflected off the dust particles. Comets usually have a second tail too. This one is called an ion or gas tail and it is formed when the ices of the nucleus are heated and turn directly into gases without passing through the liquid stage -- a process called sublimation. The gas tail forms when charged particles from the sun, called the solar wind, push the cometary gas particles directly away from the sun. The gas tail is visible because its particles glow after being excited by solar radiation.

Once comets start moving away from the sun, their activity decreases. Their tails fade and the coma disappears. They return to just the icy nucleus again. When comets' orbits eventually bring them back towards the sun, the coma and tails begin to form again.

How big are comets?

Comets come in a wide range of sizes. The largest nucleus observed is about 25 miles, or 40 kilometers, in diameter. The dust and ion tails can be enormous. Comet Hyakutake's ion tail stretched 360 million miles, or about 580 million kilometers.

How do comets form?

About 4 ½ billion years ago, our sun was just beginning to take shape as a star. As the sun got warmer and warmer, a cloud of ice and dust surrounded the early sun. As the cloud formed, it started rotating. It also slowly changed from a spherical shape to one that was flatter, shaped more like a donut with the sun in the hole. The cloud whirled around and around and the rotating cloud got flatter and denser. In the area of the cloud most distant from the sun, dust and ice particles came together and stuck to each other. Over millions of years, these particles got a little bigger. Eventually these particles grew to the size of basketballs and then cars and buildings. Finally, these particles grew to the size of city blocks. Many of these objects in the distant solar system became known as comets.

In some cases, these large particles kept sticking together, eventually forming the cores for the giant planets Jupiter, Saturn, Uranus and Neptune. A similar process occurred closer to the sun but the cloud contained very little ice. The inner planet (Mercury, Venus, Earth and Mars) formation process began when tiny pieces of dust sticking together, grew to the rocky inner planets with the leftover bits and pieces being what we now call asteroids.

What is inside a comet?

Comets are made of dust, ice and gas. Their ices are mostly water ice and their carbon-based molecules include carbon dioxide, carbon monoxide, hydrogen cyanide and methane -- some of the building blocks of life.

Where do comets come from?

Comets originate in the outer solar system. Some comets head toward the inner solar system from a region called the Kuiper Belt, which is beyond the orbit of Neptune. These comets orbit the sun in less than 200 years. Most comets come from a region that is at the extreme outer edge of our solar system. This area is called the Oort cloud. Comets that head toward the sun from here have much longer orbital periods. Comets from this area may take more than 30 million years to orbit the sun just once.

How many comets are there?

We don't know how many comets there are since most have never been seen. There could be one hundred billion in the Oort cloud. As of 2010, astronomers have discovered about 4,000 comets in our solar system.

Can comets hurt Earth?

In general, comets can be a thing of wonder to see from Earth. It is a great sight to see a comet in the night sky. Also, scientists think that comets helped bring important things like water and other materials that would help life originate and evolve on Earth.

In extremely rare cases, comets can cause problems on Earth. Most scientists believe that a very large asteroid or comet hit Earth about 65 million years ago. The resulting changes on Earth led to the extinction of the dinosaurs. Very large asteroids, as well as very large comets, could possibly cause major damage if one was to hit Earth. However, scientists believe that large impacts like the one that killed off the dinosaurs occur once very few hundred million years.

Do comets change course?

The path comets take as they orbit the sun may get altered for a few reasons. If they pass close enough to a planet, the tug of that planet's gravity can slightly change the comet's path. Jupiter, the largest planet, seems the most likely planet to alter a comet's path. Telescopes and spacecraft instruments have taken images of at least one comet -- Shoemaker-Levy 9 -- breaking up and hitting Jupiter's atmosphere. Also, comets head towards the sun and in some cases, fall right into it.

Over millions of years, most comets run into the sun or another planet, are gravitationally ejected from the solar system by a major planet or lose their ices and disintegrate during their travels through the inner solar system near the sun.